Hard-disk technology is constantly evolving. In hard drive technology, the sensor that detects magnetic information on a rotating disk has played an important role. Today's sensors are drastically different from those used even a few years ago. For example, modern sensors can detect and transmit information from recorded data at densities greater than 200 Gbit/in2 and data rates approaching 1 GHz. Advances in nanomagnetics, magnetic ultrathin films, magneto-electronics, as well as device processing, have advanced this technology. It can be expected that the future will continue to bring advances in sensor technology.
The read sensor in the recording head of hard-disk drives (HDD), based on the phenomenon of giant magnetoresistance (GMR), is an example of the commercialization of magnetic nanotechnology and spintronics. The basic magnetoresistive film can be composed of a dozen or more layers of magnetic and non-magnetic materials whose effective thickness is controlled down to sub-Angstrom level. Each of these layers directly determines or affect the magnetic and magnetotransport behavior.
From this multilayer, a working sensor and head are created after, for example, over 250 processing steps, using techniques that are near the limits of current lithography, combining insulating and conducting materials, hard magnet biasing, and magnetic shielding. The sensor is designed to fly just a few nanometers above a spinning disk at up to 15000 revolutions per minute.
The recording head has three main components: (1) the read sensor (“reader”); (2) the write transducer (“writer”), which is a microfabricated planar electromagnet with a narrow pole that creates a high density of magnetic flux in proximity to the media; and (3) the slider, which is a shaped piece of substrate (typically alumina-titanium carbide) onto which the writer and read sensor are built, and is engineered to “fly” only a few nanometers above the spinning media disk.
The subject of the present invention is the read sensor but it is understood that for any sensor, there is an appropriate combination of writer and slider which forms a coherent recording head device and, together with the chosen media, mechanical characteristics, and electronics, forms a complete recording system. The recording environment in which the head is expected to operate is first introduced, including media characteristics, magnetic interference and shielding, and signal-to-noise (SNR) considerations. These constraints put specific boundaries on the sizes, geometries, and magnetic properties which a read sensor must achieve.
The magnetic recording process utilizes a thin film transducer for the creation or writing of magnetized regions (bits) onto a thin film disk and for the detection or reading of the presence of transitions between the written bits. The thin film transducer is referred to as a thin film head. It consists of a read element, which detects the magnetic bits, and a write element, which creates or erases the bits.
FIG. 1 is a schematic of the recording process. Shown in FIG. 1 is read sensor 102, write element 104, and recording medium 106. The perpendicular write element 104 writes magnetic transitions vertically within recording medium 106 by orienting the write field perpendicular to the direction of the recording film surface. The magnetic field created by this perpendicular head returns to this element through a magnetically soft underlayer 110 within the medium, or return path. In this way the recording medium 106 lies within the write gap. The resulting perpendicular write fields can be up to two times larger than longitudinal write fields, thus enabling the perpendicular write element to write information on high coercivity media that is inherently more thermally stable. In perpendicular recording, the bits do not directly oppose each other resulting in a significantly reduced transition packing. This allows bits to be more closely packed with sharper transition signals, facilitating easier bit detection and error correction. During a read operation, read sensor 102 detects perpendicular bits 108 on recording medium 106.
In a disk recording system, successive bits are written onto the disk surface in concentric rings or tracks separated by a guard band. The head transducer is attached to a suspension, and the suspension is attached to an actuator which controls the position of the transducer in a plane above the disk surface. A specially-designed topography on the lower surface of the slider (known as the air-bearing surface or ABS) allows the head to “fly” above the rotating disk (typically 4200-15000 rpm), and controls the height of the transducer above the disk surface, typically 10 to 15 nm.
Referring now to FIG. 2, there is shown an implementation of a disk drive 200. As shown in FIG. 2, at least one rotatable magnetic disk 212 is supported on a spindle 214 and rotated by a disk drive motor 218. The magnetic recording on each disk is in the form of annular patterns of concentric data tracks on the magnetic disk 212.
At least one slider 213 is positioned near the magnetic disk 212, each slider 213 supporting one or more magnetic head assemblies 221. As the magnetic disk rotates, slider 213 moves radially in and out over the disk surface 222 so that the magnetic head assembly 221 may access different tracks of the magnetic disk where desired data are written. Each slider 213 is attached to an actuator arm 219 by way of a suspension 215.
Suspension 215 provides a spring force which biases slider 213 against disk surface 222. Each actuator arm 219 is attached to actuator 227. Actuator 227 as shown in FIG. 2 may be a voice coil motor (VCM). The VCM comprises a coil movable within a fixed magnetic field, the direction and speed of the coil movements being controlled by the motor current signals supplied by controller 229.
During operation of the disk storage system, the rotation of magnetic disk 212 generates an air bearing between slider 213 and the disk surface 222 which exerts an upward force or lift on the slider. The air bearing thus counter-balances the spring force of suspension 215 and supports slider 213 off and slightly above the disk surface by a small, substantially constant spacing during normal operation.
The various components of the disk storage system are controlled in operation by control signals generated by control unit 229. Control signals may also include internal clock signals. Typically, control unit 229 comprises logic control circuits, digital storage and a microprocessor. Control unit 229 generates control signals to control various system operations such as drive motor control signals on line 223 and head position and seek control signals on line 228. The control signals on line 228 provide the desired current profiles to optimally move and position slider 213 to the desired data track on disk 212. Write and read signals are communicated to and from write and read heads 221 by way of recording channel 225.
With reference to FIG. 3, the orientation of magnetic head 221 in slider 213 can be seen in more detail. FIG. 3 is an ABS view of slider 213, and as can be seen, the magnetic head, including an inductive write head and a read sensor, is located at a trailing edge of the slider. The above description of a typical magnetic disk storage system, and the accompanying illustrations of FIG. 1-3 are for representation purposes only. It should be apparent that disk storage systems may contain a large number of disks and actuators, and each actuator may support a number of sliders.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode, the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Magnetization of the pinned layer is usually fixed by exchange coupling one of the ferromagnetic layers (AP1) with a layer of antiferromagnetic material such as PtMn or IrMn. While an antiferromagnetic (AFM) material such as PtMn or IrMn does not in and of itself have a magnetization, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer.
In order to meet the ever increasing demand for improved data rate and data capacity, research has focused on the development of perpendicular recording systems. A traditional longitudinal recording system stores data as magnetic bits oriented longitudinally along a track in the plane of the surface of the magnetic disk. This longitudinal data bit is recorded by a fringing field that forms between a pair of magnetic poles separated by a write gap.
A perpendicular recording system, on the other hand, records data as magnetic transitions oriented perpendicular to the plane of the magnetic disk. The magnetic disk has a magnetically soft underlayer covered by a thin magnetically hard top layer. The perpendicular write head has a write pole with a very small cross section and a return pole having a much larger cross section. A strong, highly concentrated magnetic field emits from the write pole in a direction perpendicular to the magnetic disk surface, magnetizing the magnetically hard top layer. The resulting magnetic flux then travels through the soft underlayer, returning to the return pole where it is sufficiently spread out and weak that it will not erase the signal recorded by the write pole.
The advent of perpendicular recording systems has lead to an increased interest in Current perpendicular to plane (CPP) sensors, which are particularly suited to use in perpendicular recording systems, due to their ability to meet higher linear density requirements. A CPP sensor differs from a more conventional current in plane (CIP) sensor such as that discussed above in that the sense current flows through the CPP sensor from top to bottom in a direction perpendicular to the plane of the layers making up the sensor. Whereas the more traditional CIP sensor has insulation layers separating it from the shields, the CPP sensor contacts the shields at its top and bottom surfaces, thereby using the shields as leads.
One type of CPP sensor is a tunnel valve or tunnel magnetoresitive (TMR) sensor. Such sensors have a magnetic free layer and a magnetic pinned layer similar to a GMR or spin valve. The tunnel valve, however, has a thin electrically insulating barrier layer sandwiched between the free and pinned layers rather than an electrically conductive spacer layer. To meet the demands for increased sensor performance, researchers have sought to develop TMR sensors having improved performance characteristics. A theoretical improvement has been reported by constructing a TMR sensor having an Fe free layer, an Fe pinned layer and a MgO barrier formed therebetween. Such a construction has been proposed by Wulfhekel et al. in Applied Physics Letters, vol. 78, no. 4, 22 Jan. 2002.
During the manufacture of TMR sensors, however, it has been observed that damage to the sensor stack can occur with the introduction of oxygen during the formation of certain insulating layers. The damage to the sensor stack has analyzed to detrimentally affect the performance of the TMR sensor.
Therefore, there is a need for a practical CPP magnetoresistive sensor having exceptional magnetoresistive performance. More particularly, there is a need for a TMR sensor having consistent performance where such performance is not compromised by damage incurred during its manufacture.